Method and apparatus for gasifying liquid fuels and effecting a complete combustion thereof

ABSTRACT

An oil burner comprising a fuel nozzle for ejecting the liquid fuel under pressure in a forwardly diverging spray, and a conical air nozzle coaxially with and surrounding the fuel nozzle for passing a forwardly converging jet of air at a high velocity into the fuel spray so that the air particles will intersect the fuel particles to produce an intimate gaseous mixture which is then propelled against a preferably convex wall of one side of a mixture distributor which is spaced at an adjustable distance from the fuel and air nozzles and is provided with a plurality of bores through which the gaseous mixture passes to the other side of the distributor where the mixture after being ignited will burn with a blue flame. The mouths of the fuel and air nozzles are disposed within a common plane and the mouth of the air nozzle has a smaller diameter than the distributor.

United States Brodlin et a1.

atent [451 Sept. 26, 1972 [54] METHOD AND APPARATUS FOR GASIFYING LIQUID FUELS AND EFFECTING A COMPLETE COMBUSTION THEREOF [22] Filed: Oct. 14, 1970 [21] Appl. No.: 80,715

[30] Foreign Application Priority Data Oct. 14, 1969 Germany 19 57 752.3 Aug. 11, 1970 Germany ..P 20 39 872.5

52 us. Cl. ..239/500, 239/504, 239/515, I

239/524 [51] Int. Cl. ..B05b 1/26 [58] Field of Search ..239/504, 423, 424, 500, 499, 239/514, 515, 524

[56] References Cited i UNlTED STATES PATENTS 3,223,141 12/1965 Safford ..2'39/504 X 1,459,629 6/1923 Johns ..239/504 X 3,385,527 5/1968 Drewry ..239/424 595,778 12/1897 Knoop ..239/5 15 X 418,477 12/1889 Strebeck ..239/504 X 2,312,559 3/1943 Klockau ..239/504 X 1,685,108 9/1928 Tuffley ..239/5l4 X 2,044,296 6/1936 Hardgrove ..239/5l4 X FOREIGN PATENTS OR APPLICATIONS 1,157,823 7/1969 Great Britain ..,.239/424 Primary Examiner-M. Henson Wood, Jr. Assistant ExaminerEdwin Donald Grant Attorney-Craig, Antonelli & Hill [57] ABSTRACT An oil burner comprising a fuel nozzle for ejecting the liquid fuel under pressure in a forwardly diverging spray, and a conical air nozzle coaxially with and surrounding the fuel nozzle for passing a forwardly converging jet of air at a high velocity into the fuel spray so that the air particles will intersect the fuel particles to produce an intimate gaseous mixture which is then propelled against a preferably convex wall of one side of a-mixture distributor which is spaced at an adjustable distance from the fuel and air nozzles and is provided with a plurality of bores through which the gaseous mixture passes to the other side of the distributor where the mixture after being ignited will burn with a blue flame. The mouths of the fuel and air nozzles are disposed within a common plane and the mouth of the air nozzle has a smaller diameter than the distributor.

9 Claims, 8 Drawing Figures PATENTEnsEPze I972 3. 693,887

' SHEETIOFS A/ILHELM, Semina MALL; RRODMN AND $IEGFRIED WEISHAUP BY OIS, Antonebhl, Stewart. 9 ATTORNEYS Pmmmsms m2 3 s93 as? SHEET u 05 5 PATENTED SEP 26 I972 SHEET 5 [1F 5 METHOD AND APPARATUS FOR GASIFYING LIQUID FUELS AND EFFECTING A COMPLETE COMBUSTKON THEREOF The present invention relates to a method of gasifying and burning liquid hydrocarbon fuels and to a burner for such fuels which is of the pressure atomizing type and operated by an approximately stoichiometric quantity of air and the atomizing nozzle of which is surrounded by and concentric with an air guiding or blast tube the end portion of which facing the furnace chamber forms a forwardly converging air nozzle. The mouths of both nozzles of such a burner are disposed within a common plane.

Since the cross-sectional size of the mouth opening of the blast tube of any of these burners as have been previously made is relatively large, only a relatively small part of the air emerging from this outlet is mixed with the fuel'particles and it is therefore hardly possible to attain a complete combustion of the fuel particles which emerge from the fuel nozzle. In order to overcome this disadvantage and to attain a more complete combustion, it has been proposed to guide the air cur.- rent within the blast tube so as to flow with a whirling turbulence and I thereby to become more intimately mixed with the fuel emerging from the fuel nozzle. In addition it has been proposed to provide a deflector. or baffle plate in front of and at a certain distance from the fuel nozzle for the purpose of further increasing the air turbulence and for thereby attaining a still better combustion.

instead of providing such a deflector or baffle plate, it has also been proposed to improve liquid fuel burners of the pressure atomizing type by providing a distributor or combustion aid at a certain distance from the fuel nozzle and from the front end of the blast tube. Such a distributor has a convex surface facing the nozzle and according to one embodiment it consists either of a spherical body or of a body which has hemispherical part atone side facing the nozzle and a conical part on the opposite side, while according to another embodiment it consists of a hollow hemispherical element which is provided with apertures. The fuel nozzle of either of these burners is disposed within the air guiding or blast tube and its mouth is located at the rear of the mouth of the air guiding or blast tube, while the distributor or combustion aid is mounted downstream of the mouth of the blast tube and its distance from this mouth and from the fuel nozzle may be adjustable. it is the purpose of such a distributor or combustion aid to split up the liquid fuel which has not as yet been intermixed with the air and to distribute it on the surface of the distributor where it will be acted upon by the air so as to attain a better combustion. However, even with these known devices it is not possible to attain a complete and intimate mixture of the fuel particles with the particles of the compressed air since the cross-sectional size of the outlet of the air guiding or blast tube is larger than the outer diameter of the distributor upon which the unmixed fuel particles should be deposited. While it is the professed purpose of these distributors or combustion aids to stabilize the combustion, they are incapable of effecting a complete combustion, that is one burning with a blue flame and without any soot formation.

it is one of the objects of the present invention to provide a new method of combustion for attaining a blue-burning flame, and it is a further object of the invention to improve the burners of the type as mentioned in the beginning in a manner so as to insure that they will burn up the fuel which is supplied by them as completely as is practically possible.

For attaining these objects, the invention provides that the air which is to be supplied to the fuel quantity which is to be gasified and burned is provided in the form of condensed, approximately stoichiometric, concentric air current which flows at a high velocity in a direction so as to intersect the conical jet of liquid fuel flowing within the center of the air current so that due to such intersecting flow of the fuel and air the fuel particles will be more highly activated with air oxygen. A burner which is designed in accordance with the invention is provided with an air guiding or blast tube which has a conical end forming a nozzle. This conical nozzle has such dimensions that the compressed air which is supplied by a blower forms a strong concentrated blast or jet of air which has an approximately stoichiometric quantity in accordance with the quantity of liquid fuel which is to be gasified and burned. This jet of air then passes so intensively through the jet of fuel emerging from the fuel nozzle that the fuel particles will be highly activated by air oxygen and the fuel mixture which is thus produced will be gasified and after being ignited will burn with a blue flame. When the ignition is then switched off, the fuel mixture will continue to burn with such a blue flame after passing through a mixing zone without burning.

If a furnace chamber which may be of any shape and the flow and flue conditions are unknown is to be equipped with a burner according to the invention which is provided with a blast tube with a conical nozzle the mouth of which is disposed within the same plane as the mouth of the fuel nozzle and surrounds the latter, it is advisable according to another feature of the invention to provide subsequent to or downstream of the mouth of the blast tube a mixture distributor which may be of a shape similar to that of any of the distributors as previously described but has an outer diameter larger than the diameter of the mouth of the blast tube and is provided with a plurality of passages in the form of bores. Numerous tests with burners according to the invention which are provided with such distributors of different shapes have shown that each of them will burn with a steady blue flame which is absolutely odorless and free of soot and has a high CO value and a CO value far below the admissible value of 0.1 percent by volume of the flue gas.

According to another feature of the invention it is also possible to provide several distributors of different shapes in axial alignment behind each other and to make the first of these distributors following the burner nozzle of a smaller diameter than the subsequent distributor or distributors. By varying the distance of the distributors from the mouth of the fuel nozzle and also by varying their shape, it is further possible to vary the size and shape of the flame. The supporting rod or the like for the first distributor should be at least partly tubular so that the supporting rod for the second distributor may be telescopically adjustable therein. This has the advantage that the number of individual parts of which the entire burner has to be assembled will be reduced. The second distributor may also serve as an ionization element for supervising the existence of the flame. In this case, its supporting rod or the like must 'be insulated from the supporting rod of the first disignition occur which have the result that a steady flame will not be formed immediately but only after a lapse of time which may be varying in length. It is therefore a further object of the invention to design the distributor so as to insure that the flame will be formed immediately after the ignition-has occurred.

For attaining this further object, the invention provides the generally convex side of the hollow distributor facing the burner nozzle with a central depression which is smoothly curved toward the inside of the distributor and is provided with a central bore which is disposed coaxially to the fuel nozzle and has a smaller diameter than the other boresin the wall of the distributor. A distributor of such a construction has the effect that a steady flame will be formed immediately after the ignition at the inner side of the distributor and that no soot formation will occur within the central area of this distributor which might possibly occur within the central area of one of the other distributors as previously described if the furnace chamber into which the flame projects has very unfavorable com- FIG. 1 shows a longitudinal section of a burner according to the invention which is provided with a relatively long air nozzle; I

FIG. 2 shows a longitudinal section of the burner according to FIG. 1 which is additionally provided with a mixture distributor accordingto the invention;

FIG. 3 shows a longitudinal section of a burner which is provided with a relatively short air nozzle and a distributor according to the invention;

FIG. 4 shows a side view of two distributors according to the invention, one of which is mounted behind the other;

FIG. 5 shows a side view of a distributor according to the invention which is provided in the form of a hollow sphere;

FIG. 6 shows a front view of another modification of the distributor according to the invention in the form of a hollow cylinder;

FIG. 7 shows a side view of the distributor according to FIG. 6; while FIG. 8 shows a longitudinal section of a burner .according to FIG. 1 which is provided with a distributor according to a further modification of the invention.

As illustrated in FIG. 1 of the drawings, the burner according to the invention comprises a burner flange 1 in which an air guiding or blast tube 2 is secured which has a conical nozzle 3 projecting from the flange l. The

front end 4 of this air nozzle 3 is disposed within the same plane in which the front end 5 of the fuel nozzle 6 is located. The fuel nozzle 6 and the blast tube 2 and its nozzle 3 extend coaxially to each other and between them a pair of ignition electrodes 7 extend to and slightly beyond the front ends of the two nozzles 3 and 6. The air nozzle 3 is made of a conical shape so as to concentrate the air current during its flow toward the plane of the front ends of both nozzles so as to form a round strong jet of air of a high velocity which flows so intensively through and intersects the fuel particles emerging from the fuel nozzle 6 so that these particles will be thoroughly intermixed with and highly activated by the oxygen of the air current. This construction of fuel and air which is thus produced, this mixture will be gasified within a mixing zone and after being ignited will burn with ablue flame.

If by means of the burner as illustrated in FIG. 1 it is not possible to attain a steady flame in a particular furnace chamber, for example, because of the peculiarities of this chamber and of the flues of the latter, it is advisable according to the invention to provide this burner additionally with v a mixture distributor 8, as shown in FIG. 2, which is secured to a supporting rod 9 or the like which is adjustably mounted on the burner flange 1 so as to permit this distributor 8 to be adjusted to different distances from the fuel nozzle. This distributor 8 forms a part of a hollow sphere the wall of which is provided with a plurality of bores 10 forming passages and has an outer diameter D which is larger than the diameter d of the front end 4 of the air nozzle.

If desirable, it is possible as shown in FIG. 3.to make the conical air nozzle 3 of a relatively short length and also to make the partly spherical distributor 8 merely of a shallow cuplike shape. This mixture distributor is likewise provided with a plurality of bores 10.

FIG. 4 illustrates a mixture distributor of a modified construction. It comprises a partly spherical distributor element 8 similar to the distributor 8 as shown in FIG. 2 but provided with a larger number of bores 10 and behind this distributor element 8 a second distributor element which may be similar to the distributor 8 as shown in FIG. 3. The supporting rod 9' of this second distributor element 8 is mounted on the main supporting rod 9 and preferably extends into the latter which is then made at least partly of a tubular shape.

As illustrated in FIG. 5, the mixture distributor 8 may also be provided in the form of a complete hollow sphere which is also provided with a plurality of bores 10".

FIGS. 6 and 7 illustrate a mixture distributor 8 according to another modification of the invention which has also proved successful. It consists of a hollow cylinder which is provided with a plurality of bores 10".

FIG. 8 finally illustrates a mixture distributor 8 according to a further modification of the invention. On its generally convex side facing the fuel nozzle 6, this distributor 8 is provided with a central part 11 which is curved toward the inside of the distributor and is provided with a central bore 12 which is in axial alignment with the fuel nozzle 6 and has a smaller diameter than the other bores 10 of this distributor. The wall of each of these bores 10 and 12 gradually increases in diameter from the center of its length toward its inner and outer sides. If the fuel is ignited by means of the spark electrodes 7, a blue-burning flame will also be formed in this case at the inner side of the distributor 8 For supervising the existence of the flame, a second distributor element similar to that as shown in FIG. 4 may be employed as an ionization element. in this case, the supporting rod 9 of this second distributor element 8 should be insulated from the supporting rod 9 of the first distributing element 8 In place of such a second distributor it is also possible to provide an ionization element of a conventional type.

Although this invention has been illustrated and described with reference to the preferred embodiments thereof, it should be understood that it is in no way limited to the details of such embodiments but is capa ble of numerous modifications within the scope of the appended claims. In any embodiment of the invention it is, however, the most important feature that the front end of the blast tube is made of a conical shape so as to serve as an air nozzle which is designed to produce an intensive air current of a high velocity and of a shape and volume which may beaccurately predetermined.

What we claim is:

1. A burner for gasifying liquid fuels and burning th same with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distibutor having a convex wall facing said nozzles and spaced downstream therefrom and having a center disposed in axial alignment with said nozzles, said distributor having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of said distributor.

2. A' burner as defined in claim l, in which said distributor has a shape forming at least a part of a sphere, and adjustable means for mounting said distributor so as to permit its distance from said nozzles to be varied.

3. A burner as defined in claim 1, in which at least two of said distributors are mounted behind and spaced from each other and disposed in axial alignment with said nozzles.

4. A burner as defined in claim 3, in which the second of said distributors spaced at a greater distance from said nozzles than the first distributor has a larger diameter than said first distributor.

5. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least one hollow mixture distributor in the form of a tubular member having a longitudinal axis extending at a right angle to the axis of said nozzles, said distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, having a plurality of round borelike passages extending therethrough, and having an outer diameter larger than thediameter of the mouth of said air nozzle, and adjustable means for mounting said distributor so as to permit its distance from said nozzles to be varied.

6. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least two hollow mixture distributors being mounted behind and spaced from each other, each having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzle, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of each of said distributors, and a longitudinally adjustable first supporting rod secured to the first of said distributors, and a second support rod secured at one end to the second distributor and adapted to be secured with its other end to said first supporting rod and to be longitudinally adjusted relative to said first supporting rod so that said distributors are adjustable to different distances from each other and from said nozzles.

7. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least two hollow mixture distributors being mounted behind and spaced from each other, each having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of each of said distributors, and longitudinally adjustable first and second supporting rods for said first and second distributors, respectively, said second distributor being electrically insulated from said first distributor and adapted to serve as an ionization element for supervising the existence of a flame on said distributors.

8. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, and have ing a plurality of round borelike passages extending therethrough, said mouth of said air noule having a diameter smaller than the diameter of said distributor, and said distributor also having a substantially hemispherical hollow shape, a central part of its wall being curved inwardly toward the open side of said distributor, said central part having one round borelike passage in axial alignment with said fuel nozzle, said one passage having a smaller diameter than the other borelike passages in the other parts of said distributor.

9. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a

conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with saidnozzles, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of said distributor, and 'each of said borelike passages converging uniformly from both ends toward the center of its axial length. 

1. A burner for gasifying liquid fuels and burning the same with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distibutor having a convex wall facing said nozzles and spaced downstream therefrom and having a center disposed in axial alignment with said nozzles, said distributor having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of said distributor.
 2. A burner as defined in claim 1, in which said distributor has a shape forming at least a part of a sphere, and adjustable means for mounting said distributor so as to permit its distance from said nozzles to be varied.
 3. A burner as defined in claim 1, in which at least two of said distributors are mounted behind and spaced from each other and disposed in axial alignment with said nozzles.
 4. A burner as defined in claim 3, in which the second of said distributors spaced at a greater distance from said nozzles than the first distributor has a larger diameter than said first distributor.
 5. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least one hollow mixture distributor in the form of a tubular member having a longitudinal axis extending at a right angle to the axis of said nozzles, said distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, having a plurality of round borelike passages extending therethrough, and having an outer diameter larger than the diameter of the mouth of said air nozzle, and adjustable means for mounting said distributor so as to permit its distance from said nozzles to be varied.
 6. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least two hollow mixture distributors being mounted behind and spaced from each other, each having a convex wall facing said nozzles and spaced downstream tHerefrom, having a center disposed in axial alignment with said nozzle, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of each of said distributors, and a longitudinally adjustable first supporting rod secured to the first of said distributors, and a second support rod secured at one end to the second distributor and adapted to be secured with its other end to said first supporting rod and to be longitudinally adjusted relative to said first supporting rod so that said distributors are adjustable to different distances from each other and from said nozzles.
 7. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, at least two hollow mixture distributors being mounted behind and spaced from each other, each having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of each of said distributors, and longitudinally adjustable first and second supporting rods for said first and second distributors, respectively, said second distributor being electrically insulated from said first distributor and adapted to serve as an ionization element for supervising the existence of a flame on said distributors.
 8. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of said distributor, and said distributor also having a substantially hemispherical hollow shape, a central part of its wall being curved inwardly toward the open side of said distributor, said central part having one round borelike passage in axial alignment with said fuel nozzle, said one passage having a smaller diameter than the other borelike passages in the other parts of said distributor.
 9. A burner for gasifying liquid fuels and burning the fuels with a blue flame comprising a fuel nozzle adapted to eject a conically diverging spray of fuel, a conically converging air nozzle coaxially to and surrounding said fuel nozzle and having a mouth disposed within the same plane as the mouth of said fuel nozzle and adapted to eject a concentrated conically converging current of air at a high velocity so that the particles of said air current intersect the particles of said fuel spray at a high velocity, and at least one hollow mixture distributor having a convex wall facing said nozzles and spaced downstream therefrom, having a center disposed in axial alignment with said nozzles, and having a plurality of round borelike passages extending therethrough, said mouth of said air nozzle having a diameter smaller than the diameter of said distributor, and each of said borelike passages convergiNg uniformly from both ends toward the center of its axial length. 